Plastic-lined metallic pipe for conveying drinking-water and the connectors for the same

ABSTRACT

A plastic-lined metallic pipe for conveying liquid or gas includes a continuous inner plastic sleeve and an outer seamless metallic pipe. The outer diameter of the plastic sleeve is circumferentially melt-bonded to the inner diameter of the outer pipe. The plastic sleeve constitutes an uninterrupted lining for the metallic pipe. Also disclosed is a connector used for connecting the plastic metallic pipe. The connector includes a body, a cap, and three washers. The body has a section of thread in its outer wall for coupling with the cap which has a section thread in its inner wall. In operation, first insert the pipe into the cap, then insert the rubber washer, the metallic washer, and spring washer into the body, and then screw the cap into the body, such that the pipe is locked with the body.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention generally relates to the field of designing andmanufacturing of metallic pipe for conveying drinking-water. Moreparticularly the invention relates to a pipe with an outer metallic pipeand an inner plastic pipe suitable for conveying drinking-water and theconnectors for the pipe.

2. Description of the Related Art

Zecchin, et al, in PCT/EP98/08182, disclosed a composite pipe made ofmetal-plastic for hydro-thermo-sanitary plants and method for theproduction thereof. The composite pipe consist of an internal metalpipe, an outer pipe made of a plastic suitable for conveyingdrinking-water, and an intermediate layer having a bi-adhesive function.The outer pipe and the intermediate layer are both extruded onto theinner metal pipe. The material for the outer pipe and the intermediatelayer preferably contains an anti-oxidizer additive for the pipe metal.The primary purpose of Zecchin's invention is to prevent corrosions dueto the action of local leakage currents and/or chemical corrosion causedby acid substances released from building materials like concrete of thefloors in which the pipes are embedded.

David A. Shotts, et al, in U.S. Pat. No. 5,152,323, disclosed aplastic-lined metal pipe which includes an outer metallic pipe having alongitudinal weld and an inner plastic sleeve melt-bonded to themetallic pipe. The metallic pipe is of a size to be in mechanicalengagement with the sleeve around the entire circumference of thesleeve. The pipe is manufactured using an extruder, for providing aplastic sleeve, in conjunction with a continuous roll-forming tubingmill production line, for forming and treating the metallic outer tube,so that the lined pipe is made using a continuous process. The plasticsleeve is caused to not collapse during the manufacturing process andthe sleeve and pipe are firmly locked together. The pipe may be made byapparatus for continuously manufacturing plastic-lined metal pipeincluding an extruder for providing a plastic sleeve and a continuousroll-forming tubing mill production line. The production line operatesto sequentially deform a substantially flat steel strip to a generallytubular configuration and includes an electric resistance welder or highfrequency welder for continuously welding the lateral edges of themoving strip to complete the pipe. A plastic sleeve is fed into theabout-to-be-formed metallic pipe upstream of the welder. This plasticsleeve has an outside diameter slightly smaller than the inside diameterof the metallic pipe in the as-formed condition of the sleeve and thesleeve maintains its generally tubular configuration from the time it isinserted into the metal pipe until the manufacture is completed. Theproduction line also includes a station for reducing the inside diameterof the metallic pipe to substantially the outside diameter of theplastic sleeve thereby locking the two components together.

Alexander Esser in U.S. Pat. No. 6,575,197 disclosed a double-layer pipefor fluidic transport of abrasive solids. The pipe includes a hardenedinner pipe portion of steel; an outer pipe portion of weldable steel;terminal coupling collars made of weldable steel; and a heat-insulatinglayer disposed between the inner pipe portion and the outer pipeportion. The pipe includes a heat-insulating layer between the innerpipe portion and the outer pipe portion to form a barrier during heatingof the inner pipe portion to thereby prevent heat from dissipating to asignificant degree from the inner pipe portion to the outer pipeportion. The applied heat remains in the inner pipe portion so that theinner pipe portion, especially when thin pipe walls are involved, can beheated evenly within a very narrow temperature window. When the innerpipe portion is then quenched, a markedly even hardness is realized overthe entire circumference as well as length of the inner pipe portion.The heat-insulating layer disposed between the inner pipe portion andthe outer pipe portion may be formed by a coating made of a combustiblenon-metallic material and applied upon the outer surface of the innerpipe portion and/or the inner surface of the outer pipe portion.

John Werner in U.S. Pat. No. 4,507,842 disclosed an improved method forsealing and protecting a plastic lined pipe joint. The improved plasticcollar seal is placed in the interior of a pipe joint formed betweenfirst and second externally threaded plastic lined pipe sections whichare to be held together by an internally threaded pipe collar. Theimproved plastic collar seal is a hollow, open ended, cylindricalplastic collar seal presized to concentrically fit within the plasticliner of the sections of plastic lined pipe with minimum concentrictolerance. The open ends of the plastic collar seal are internallybeveled and the plastic collar seal is equipped with external O-ringgrooves and O-rings at each of the ends. The collar seal is furtherprovided with an external ledge or ridge molded concentrically to thecollar seal between the O-rings wherein the ledge is adapted to fitwithin the internally threaded collar and rest on the end of the linedpipe. This ledge is positioned such that the distance from at least oneend of the collar seal to the ledge exceeds the length of the internallythreaded collar. In operation, the collar is threaded on the firstplastic lined pipe section and the improved plastic collar seal isinserted within the pipe section and collar with the ledge coming torest on the end of the pipe section leaving the other end of the plasticcollar seal extending beyond the collar. The second section of plasticlined pipe can then be easily threaded into the internally threadedcollar, over the collar seal, thus forming a tight pipe joint with thecollar seal within the joint.

John J. Hunter in U.S. Pat. No. 4,709,946 disclosed a pipe joint whichsecures two lined pipe sections together and seals the juncture of thepipe sections against the migration of fluids through the joint andmethods for forming the pipe joint. The method includes the steps ofaxially aligning first and second pipe sections; securing the first pipesection to the second pipe section; overlapping the liners of the firstand second pipe sections; and compressing the liners of the first andsecond pipe sections where overlapped to seal the joint againstmigration of pressurized fluids through the joint. The pipe jointprovided by this method includes, in one embodiment, first pipe meanshaving a fluid impervious first liner mounted on the interior thereof;second pipe means having a fluid impervious second liner mounted on theinterior thereof; means for securing said first pipe means in axialalignment with said second pipe means wherein said first liner and saidsecond liner are partially overlapped; and means for compressing saidfirst and second liners were overlapped to seal the pipe joint formed atthe juncture of said first and second pipe means for the transmission offluids there through.

What is desired is a plastic-lined metallic pipe with a food-gradenon-poisonous inner plastic lining pipe and an outer metallic pipe madeof high strength, light-weight and corrosion resistant aluminum alloy.

SUMMARY OF THE INVENTION

The plastic-lined metallic pipe for conveying liquid or gas includes acontinuous inner plastic sleeve or pipe and an outer seamless metallicpipe. The outer diameter of the inner plastic sleeve iscircumferentially melt-bonded to the inner diameter of the outermetallic pipe. The inner plastic sleeve constitutes an uninterruptedlining for the outer metallic pipe. The inner plastic sleeve is formedof a non-poisonous plastic, such as polypropylene, suitable forconveying drinking-water. The outer metallic pipe is formed of aluminumalloy.

The method for a plastic-lined metallic pipe for conveying liquid or gasincludes at least the steps of: (1) heating a metallic pipe evenly to afirst temperature at which the metallic pipe has a higher tensility; (2)cooling a plastic pipe evenly to a second temperature which is lowerthan the regular storage temperature; (3) inserting the plastic pipeinto the metallic pipe to constitute an inner sleeve; (4) drawing themetallic pipe from its two ends for a specific length; and (5) coolingthe sleeved pipe to a storage temperature.

Also disclosed is a connector used for connecting plastic-lined metallicpipes. The connector includes the following components:

1. at least one metallic member as the body having a first steppedcylindrical inner space and a section of thread in its outer wall, thefirst stepped cylindrical inner space including a first cylindricalspace as a conveying channel, a second cylindrical space for holding apipe to be connected to the body, a tapered cylindrical space, and athird cylindrical space, the first cylindrical space's diameter beingidentical with the pipe's inner diameter and the second cylindricalspace's diameter being slightly larger than the pipe's outer diameter,the tapered cylindrical space merging the second cylindrical space withthe third cylindrical space;

2. a cap or nut having a second stepped cylindrical inner space andhaving a section of thread in its inner wall for coupling with the body,the second stepped cylindrical inner space including a fourthcylindrical space for coupling with the body and a fifth cylindricalspace in the cap's end wall for holding the pipe, the fifth cylindricalspace's diameter being slightly larger than the pipe's outer diameter;

3. a tapered rubber washer for filling the gap between the pipe and thewall of the second cylindrical space and filling the tapered cylindricalspace of the body;

4. a metallic washer for pressing the rubber washer; and

5. a “c” type spring washer for locking the pipe when the cap is beingcoupled with the body, the spring washer having a rough inner surface ora section of male thread for enforcing the locking.

In the operation to connect the pipe to the body, the followingassembling order is required: inserting one end of the pipe through thecap from the cap's end wall, then through the “c” type spring washer,the metallic washer and the tapered rubber washer with its wider endfacing the metallic washer, then inserting the one end of the pipe intothe second cylindrical space of the body, then sliding the taperedrubber washer and the metallic washer into the third cylindrical spaceof the body, then slightly squeezing the spring washer and inserting itinto the third cylindrical space of the body, and then screwing the capin.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the cross-sectional views of theplastic-lined metallic pipe according to the preferred embodiment of thepresent invention;

FIG. 2 is a schematic diagram showing the process of inserting the innerplastic pipe into the outer metallic pipe;

FIG. 3 and FIG. 4 are schematic diagrams showing the process of drawingthe outer metallic pipe to make the outer metallic pipe being bond tothe inner plastic pipe;

FIG. 5 is a schematic diagram showing the process of cooling the outermetallic pipe to make the outer metallic pipe being firmly bond to theinner plastic pipe;

FIG. 6 is a block flow diagram showing the process of making theplastic-lined metallic pipe;

FIG. 7 is a schematic front-view diagram illustrating the components ofthe connector according to the present invention;

FIG. 8 is a schematic cross sectional view diagram illustrating thecomponents of the connector and the assembling order;

FIG. 9 is a schematic diagram illustrating the connector's innerstructure when the cap is screwed in;

FIG. 10 is a schematic diagram illustrating the side view and the crosssectional views of the connector's body according to a typicalembodiment of the present invention;

FIG. 11 is a schematic diagram illustrating the side view and the crosssectional views of the tapered rubber washer according to a typicalembodiment of the present invention;

FIG. 12A is a schematic diagram illustrating the side view and the crosssectional views of the stainless metal washer according to a typicalembodiment of the present invention;

FIG. 12B is a schematic diagram illustrating the side view and the crosssectional views of the stainless metal washer according to anotherembodiment of the present invention;

FIG. 12C is a schematic diagram illustrating the side view and the crosssectional views of the stainless metal washer according to anotherembodiment of the present invention;

FIG. 13A is a schematic diagram illustrating the side view and the crosssectional views of the “c” type spring washer according to a typicalembodiment of the present invention;

FIG. 13B is a schematic diagram illustrating the side view and the crosssectional views of the “c” type spring washer according to anotherembodiment of the present invention;

FIG. 14A is a schematic diagram illustrating the side view and the crosssectional views of the cap according to a typical embodiment of thepresent invention;

FIG. 14B is a schematic diagram illustrating the side view and the crosssectional view of the cap according to another embodiment of the presentinvention;

FIG. 15A is a schematic diagram illustrating the inner structure of theconnector before the cap is entirely screwed in;

FIG. 15B is a schematic diagram illustrating the inner structure of theconnector after the cap is entirely screwed in;

FIG. 16A is a schematic diagram illustrating a stainless steel washerwith an extruding edge facing the end wall of the cap;

FIG. 16B is a schematic diagram illustrating a spring washer with atapered edge facing the connector's body; and

FIG. 16C is a schematic diagram illustrating the inner structure of theconnector when the cap is being screwed in.

DETAILED DESCRIPTION OF THE INVENTION

The plastic-lined metallic pipe according to the typical embodiment ofthe present invention, as shown in FIG. 1, consists of an outer metallicpipe 11 and an inner plastic pipe 12 suitable for conveyingdrinking-water. There is no intermediate layer between the inner plasticpipe and the outer metallic pipe. In other words, the inner plastic pipeand the outer metal pipe are firmly bond or melt-bond.

The outer seamless pipe is made of alloy material with the features ofhigh mechanical strength, non-aging, corrosion resistance and shockresistance.

The inner pipe is made of food-grade non-poisonous Polypropylene (PP),which is pollution free, corrosion resistant and low resistance inliquid.

FIG. 3, FIG. 4 and FIG. 5 are cross sectional view diagrams illustratingthe process for making the plastic-lined metallic pipe and FIG. 6 is ablock flow diagram illustrating the primary steps of the process:

Step 21: Place the alloy outer pipe 11 in a mould with a diameterslightly larger than the outer pipe's outer diameter and warm the outerpipe to a temperature (t1) at which the alloy has a higher tensility.

Step 22: Cool the inner plastic pipe 12 to a temperature (t2) which isusually lower than the normal storage temperature. The outer diameter ofthe inner pipe 12 is slightly smaller than the inner diameter of theouter pipe 11 so that the inner pipe 11 can be inserted into the outerpipe during the process. At the temperature t2, the inner pipe'sdiameter is at its shortest point. At a temperature higher than t2, theinner pipe's diameter may increase due to the mechanism of heatexpansion.

Step 23: As shown in FIG. 2, insert the inner plastic pipe 12 into theouter alloy pipe 1 1.

Step 24: Draw from both ends of the outer pipe along the axial directionto extend it for a specific length. As shown in FIG. 3, L1 representsthe initial length prior to the drawing, and as shown in FIG. 4, L2represents the length subsequent to the drawing. The tensile ratio maybe 0.1˜1% (L2−L1=0.1˜1% L1) depending upon the materials used. As shownin FIG. 5( a), the outer pipe 11 contracts during the drawing. Becausethe initial temperature of the inner plastic pipe 12 is lower than theouter pipe's temperature, the inner plastic pipe 12 is being warmed upduring the process and, as shown in FIG. 5( b), it expands with thewarming. Thus, after the drawing and the accompany heat-transfer, theinner pipe 12 and the outer pipe 11 are tightly merged together as shownin FIG. 5( c).

Step 25: Cool the outer pipe 11 to a normal temperature (t3). Duringthis cooling phase, as shown in FIG. 5( a), the outer pipe 11 furthercontracts such that the pipes are integrated to a melt-bond condition ora substantially melt-bond condition. Note that because the temperaturet2 is lower than t3, the cooling process for the outer pipe 11 isactually a warming-up process for the inner pipe 12, and thus, as shownin FIG. 5( b), it further expands during this phase.

The plastic-lined pipe according to this invention is mechanicallysuperior to stainless steel pipe and copper pipe, while its price is 20%lower than the price of copper pipe.

The outer surface of the plastic-lined pipe is finished by dull polishand antioxidant process to create a sense of metal quality, giving afeeling as bright, elegant and fashionable.

The present invention also includes a connector for connecting theplastic-lined pipes. The connector adopts chrome-plating Al pipe, silicarubber washer and a stainless steel lock-loop design. When installing,just use a regular steel-saw to cut the pipe in proper length, theninsert it into the connector's body and tighten the sealing cap. Theoperation is simple, convenient and fast.

FIG. 7 is a schematic front-view diagram illustrating the components ofthe connector according to the present invention and FIG. 8 is aschematic cross sectional view diagram illustrating the components ofthe connector and the assembling order thereof. The connector may have atwo-way base or a multiple way base. But the connecting mechanism issame. As illustrated in FIG. 7, the connector has a three-way base 30,which has three identical holding bodies such as 31. The holding body 31has a section of thread 41 around its outer surface and a steppedcylindrical inner space for holding the pipe 29, the rubber washer 32,the stainless metal washer 33, and the “c” type spring washer 34. Thespring washer 34 may have a rough inner surface or a section of malethread for enforcing the locking. Another member of the connector is thecap 35 which has a section of inner thread 42 for coupling with theholding body 31 and has an end wall 43 with a cylindrical hole forholding the pipe 29.

To connect the pipe 29 to the base 30, first insert the pipe 29 into thecap 35, then the “c” type spring washer 34, then the stainless metalwasher 33, then the rubber washer 32, and then insert the pipe 29 intothe holding body 31 to reach the supporting end 44 as shown in FIG. 8and FIG. 9. Then, slide the rubber washer 32 and the stainless metalwasher 33 into the body 31, and insert the spring washer 34 into thebody 31 so that the spring washer 34 has an elastic contact with theinner wall of the body 31. Then, screw the cap 35 toward the body 31. Bycoupling the cap 35 with the body 31, the washers 32-34 are squeezedtoward the body 31 by the end wall 43 of the cap 35. The deeper the cap35 is screwed in, the tighter the “c” type spring washer 34 is contactedwith the pipe 29. In this way, the pipe 29 is firmly connected with thebody 31 as shown in FIG. 9.

FIG. 10 is a diagram illustrating the side view and the cross sectionalviews of the body 31. The first inner cylindrical space 45 is forconveying water or other liquid or gas. The diameter of the firstcylinder 45 is identical with the inner diameter of the pipe 29 suchthat when the liquid or gas is conveyed smoothly from the cylinder 45 tothe pipe 29 or vice versa. The second cylindrical space 46 is forholding the pipe 29. The diameter of the cylinder 46 is slightly largerthan the diameter of the pipe 29 such that the pipe 29 can be insertedinto the cylinder 46. The bottom of the second cylinder 47 adjacent tothe first cylinder 45 constitutes the supporting end 44, which is aplain ring surface to be contacted by the cross-cut plain ring surfaceof the pipe 29. In its front opening, the body 31 has a thirdcylindrical space 48 for holding the stainless metal washer 33 and the“c” type spring washer 34. The diameter of the third cylindrical space48 is larger than the diameter of second cylindrical space 46. Further,between the second cylinder 46 and the third cylindrical space is atapered cylindrical space 47 which merges the third cylindrical space 48with the second cylindrical space 46.

FIG. 11 is a diagram illustrating the side view and the cross sectionalviews of the rubber washer 32. The rubber washer 32 has a taperedcylindrical structure with a thinner front and a thick back. Because itis made of an elastic material, its inner diameter 49 is slightlysmaller than the outer diameter of the pipe 29 such that the rubberwasher 32 is tightly covered around the pipe 29 when it is slipped on.The tip 50, i.e. the thinner front edge, of the rubber washer 32 istoward the body 31. The tip 50 is inserted into the gap between the pipe29 and the wall of the cylinder space 46. When the cap 35 is crewed in,the rubber washer 32 is pressed and the gap between the pipe 29 and thewall of the cylinder space 46 is sealed, such that the liquid or gasconveying through the connector and the pipe does not leak out.

FIG. 12A is a diagram illustrating the side view and the cross sectionalviews of the stainless metal washer 33 according to a typical embodimentof the present invention. Referring back to FIG. 9, when the cap 35 iscrewed in, the stainless metal washer 33 presses the rubber washer 32against the body 31 to avoid leak, and at the same time, the stainlessmetal washer 33 presses the spring washer 34 against the end wall 43 ofthe cap 35 to force the “c” type spring washer 34 to lock around thepipe 29.

FIG. 12B is a diagram illustrating the side view and the cross sectionalviews of the stainless metal washer 33 according to another embodimentof the present invention.

Yet FIG. 12C is a diagram illustrating the side view and the crosssectional views of the stainless metal washer 33 according to anotherembodiment of the present invention.

FIG. 13A is a diagram illustrating the side view and the cross sectionalviews of the “c” type spring washer 34 according to a typical embodimentof the present invention. Referring back to FIG. 9, when the cap 35 iscrewed in, the end wall 43 of the cap 35 presses the spring washer 34against the stainless metal washer 33 which further presses the rubberwasher 32 against the body 31 to avoid leak, and at the same time, thestainless metal washer 33 presses the spring washer 34 against the endwall 43 of the cap 35 to force the spring washer 34 to lock around thepipe 29. When the pressure is released by screwing out the cap 35, the“c” type spring washer 34 unlocks from the pipe 29 and returns to itsoriginal shape.

FIG. 13B is a diagram illustrating the side view and the cross sectionalviews of the “c” type spring washer 34 according to another embodimentof the present invention. The “c” type spring washer 34 responds both tothe force from the axial direction. It also responds to the pressurefrom the surrounding. Before the force is applied on it, it can slidealong the pipe 29. However, when the force is applied on it, the gap 51is getting smaller and smaller such that the spring washer 34 locks onthe pipe 29 tightly. When the force is released, the spring washer 34unlocks from the pipe 29 and returns to its original shape with the gap51.

FIG. 14A is a diagram illustrating the side view and the cross sectionalviews of the cap 35 according to a typical embodiment of the presentinvention. The cap 35 includes a larger cylindrical space 52 and asmaller cylindrical space 53. The larger cylindrical space 52 isenclosed with a section of thread 42 for coupling with the holding body31. The smaller cylindrical space 53 is for the pipe 29 to be insertedthere through. The diameter of the smaller cylindrical space 53 shouldbe slightly larger than the diameter of the pipe 29. The depth 54 of thelarger cylindrical space 52 should be smaller than the total thicknessof the spring washer 34, the stainless metal washer 33, and the rubberwasher 32, such that when the cap 35 is screwed in, the end wall 43 ofthe cap 35 may provides sufficient pressure on these three washersrequired to avoid leak and have the spring washer 34 locked around thepaper 29.

FIG. 14B is a schematic diagram illustrating the side view and the crosssectional view of the cap 35 according to another embodiment of thepresent invention. In this embodiment, the end wall 43 of the cap 35 hasa cylinder 55 extruding toward the body 31. The cylinder 55 is used tostrengthen the end wall's support to the “c” spring washer 34. The depth56 from the front of the cylinder 55 to the front of the cap 35 shouldbe smaller than the total thickness of the spring washer 34, thestainless metal washer 33, and the rubber washer 32, such that when thecap 35 is screwed in, the front of the cylinder 55 of the cap 35 mayprovides sufficient pressure on these three washers required to avoidleak and have the spring washer 34 locked around the paper 29.

FIG. 15A is a schematic diagram illustrating the inner structure of theconnector before the cap 35 is entirely screwed in. FIG. 15B is aschematic diagram illustrating the inner structure of the connectorafter the cap 35 is entirely screwed in. When the cap 35 is screwed in,the force from the cylinder 55 makes the washers tightly stackedtogether. Because the rubber washer 32 is elastic, when it is pressed,the surrounding gap between the pipe 29 and the body 31 is sealed, andthus leak is avoided. In addition, because of the pressure from thestainless steel washer 33 and the pressure from the cylinder 55, thespring washer 34 is squeezed, and thus it tightly locks around the pipe29.

FIG. 16A is a schematic diagram illustrating a stainless steel washer 71with an extruding edge 72 facing the end wall of the cap 35. FIG. 16B isa schematic diagram illustrating a spring washer 73 with a tapered edge74 facing the connector body 31. FIG. 16C is a schematic diagramillustrating the inner structure of the connector when the cap 35 isbeing screwed in. The extruding edge 72 of the stainless steel washer 71and the tapered edge 74 of the spring washer 73 are designed in such amanner that when they are squeezed together, the extruding edge 72 ofthe steel washer 71 applies a force along the axial direction to thetapered edge 74, and the tapered edge 74 changes the force into a forceconcentrating toward the center of the spring washer 34. Theconcentrating force makes the spring washer 34 locked around the pipe29.

Although the invention has been described with reference to at least onespecific embodiment, this description is not meant to be construed in alimiting sense. Various modifications of the disclosed embodiment,alternative embodiments or other equivalent solutions of implementingthe disclosed pipe and connector will become apparent to those skilledin the art upon reference to the description of the invention. It istherefore contemplated that such modifications, equivalents, andalternatives can be made without departing from the spirit and scope ofthe present invention as defined in the appended claims.

1. A plastic-lined metallic pipe for conveying liquid or gas,comprising: a continuous inner plastic sleeve; and an outer seamlessmetallic pipe, the outer diameter of said plastic sleeve beingcircumferentially melt-bonded to the inner diameter of said outer pipe,said plastic sleeve constituting an uninterrupted lining for saidmetallic pipe.
 2. The plastic-lined metallic pipe of claim 1, whereinsaid plastic sleeve is formed of a non-poisonous plastic suitable forconveying drinking-water.
 3. The plastic-lined metallic pipe of claim 2,wherein said plastic sleeve is formed of polypropylene.
 4. Theplastic-lined metallic pipe of claim 2, wherein said metallic pipe isformed of aluminum alloy.
 5. A method for a plastic-lined metallic pipefor conveying liquid or gas, comprising steps of: heating a metallicpipe evenly to a first temperature; cooling a plastic pipe evenly to asecond temperature; inserting the plastic pipe into the metallic pipe toconstitute an inner sleeve; drawing the metallic pipe from its two ends;and cooling the sleeved pipe to a storage temperature.
 6. The method ofclaim 5, wherein said plastic pipe is formed of a non-poisonous plasticsuitable for conveying drinking-water.
 7. The method of claim 6, whereinsaid plastic pipe is formed of polypropylene.
 8. The method of claim 7,wherein said metallic pipe is formed of aluminum alloy.
 9. A connectorused for connecting pipes, comprising: at least one body having a firststepped cylindrical inner space and a section of thread in its outerwall, the first stepped cylindrical inner space including a firstcylindrical space as a conveying channel, a second cylindrical space forholding a pipe to be connected to the body, a tapered cylindrical space,and a third cylindrical space, the first cylindrical space's diameterbeing identical with the pipe's inner diameter and the secondcylindrical space's diameter being slightly larger than the pipe's outerdiameter; a cap having a second stepped cylindrical inner space andhaving a section of thread in its inner wall for coupling with the body,the second stepped cylindrical inner space including a fourthcylindrical space for coupling with the body and a fifth cylindricalspace in the cap's end wall for holding the pipe; a tapered rubberwasher for filling the gap between the pipe and the wall of the secondcylindrical space; a metallic washer for pressing the rubber washer; anda “c” type spring washer for locking the pipe when the cap is beingcoupled with the body; wherein when connecting the pipe to the body, thefollowing assembling order is complied: inserting one end of the pipeinto the cap from the cap's end wall, then into the spring washer, themetallic washer and the rubber washer, then inserting the one end of thepipe into the second cylindrical space of the body, then sliding therubber washer and the metallic washer into the third cylindrical spaceof the body, then slightly squeezing the spring washer and inserting itinto the third cylindrical space of the body, and then screwing the capin.
 10. The connector of claim 9, wherein the metallic washer has acylindrical extruding member facing the spring washer, such that whenthe cap is screwed in, the extruding member applies a force to thespring washer to make the spring washer squeeze and lock around thepipe.
 11. The connector of claim 10, wherein the spring washer has atapered cylindrical surface facing the metallic washer, such that whenthe cap is screwed in, the tapered cylindrical surface transforms theforce from the extruding member into a concentrating force required tosqueeze and lock the spring washer.
 12. The connector of claim 9,wherein the cap's end wall has a cylindrical extruding member facing thespring washer, such that when the cap is screwed in, the extrudingmember applies a force to the spring washer to make the spring washersqueeze and lock around the pipe.
 13. The connector of claim 12, whereinthe spring washer has a tapered cylindrical surface facing the extrudingmember, such that when the cap is screwed in, the tapered cylindricalsurface transforms the force from the extruding member into aconcentrating force required to squeeze and lock the spring washer. 14.The connector of claim 9, wherein the spring washer has a rough innersurface for locking around the pipe.
 15. The connector of claim 9,wherein the spring washer has a section of thread in its inner surfacefor locking around the pipe.